Word length can be extremely revealing. A I'd suggest covering the entire document. Plus a large (& random) number of junk pages which you prepend and append to the document. One pixel per page should be fine.
It's still not a good idea. Analysts may attempt to de-cloak the word with varying degrees of success by analyzing the width of the redacted word against font character widths and using techniques from the study of Natural Language Processing to identify likely word candidates that fit in context (e.g., with respect to the rest of the sentence or document).
